Requirements for the biomass and production process
The prerequisite for the economical use and an efficient downstream process of the algae biomass is a high lipid content. Furthermore, the spectrum of fatty acids should have a high proportion of saturated and monounsaturated fatty acids, as polyunsaturated fatty acids reduce the storage stability of the algae oil. The enrichment of fatty acids in the form of triglycerides can be induced by a nitrogen limitation of the microalgae culture. The triglycerides are deposited inside the cells as storage molecules. In laboratory experiments at the Fraunhofer IGB we were able to achieve lipid contents of up to 70 percent [w/w] under permanent artificial light.
When producing algae as energy source, it is necessary to transfer this process into an outdoor cultivation process. Here the challenge is to establish a process that runs well and remains stable under varying outdoor conditions, and also generates biomass with a high lipid content. It has to be taken into account that the recurrent day-night rhythm and changing weather conditions result in a variable process temperature and different light intensities.
Outdoor plant for lipid production
In 2010 and 2011 we operated an outdoor pilot plant with five south-facing 30-liter flat-panel airlift reactors to characterize the lipid production process with the microalgae Chlorella vulgaris. A two-stage batch process was established. Biomass was produced in a first growth phase from four to seven days with an optimum supply of nutrients. This was followed by the lipid production phase, in which the algae cells enriched lipids by means of nitrogen and phosphate limitation. The focus of the investigation was to maximize the lipid content of the algae and to determine a quantitative correlation between the relative light intensity and the biomass concentration in the reactor. The relative light intensity describes the ratio of light input to biomass concentration in the reactor and is specified as Einstein (1 mole of photons) per gram of dry mass and day.